The short answer is that all cells come from
other cells. Cells can only be formed when
another cell divides to make 2 "daughter cells"
that have the same DNA. Sometimes 2 cells will
join to form one, such as a fertilized egg cell.
Their DNA is combined in the new cell.

Of course, you might be wondering where the
first cell came from. A lot of people wonder
about that. Since the first cells formed billions
of years ago, and single cells don't leave
fossils, that's a tough question. It's possible
that there were a few different types of cells
that formed around the same time. There's
evidence that modern plant and animal cells are a
result of two or more kinds of cells joining
together.